1999: Investigation of Aerosol Variability and Trends in NYC

Researchers: Andre Cassell, Barbara Carlson, Andy Lacis, Brian Cairns, Kevin Finnerty, Christine Fleming, Rashelle Cross, Robert Gandolfo, Susane Colasanti, Ingrid Gonzalez, Jessica Morales, and Jenny Solis
Campus-based: City College of New York team led by Fred Moshary; LaGuardia Community College team led by James Frost; Medgar Evers College team led by Robert Craig.

Introduction

Aerosols are tiny particles suspended in the air. These particles may be solid (irregularly shaped) or liquid (spherical); they range in size from 0.01 to several tens of microns. For example, cigarette smoke particles are in the middle of this size range while cloud drops are typically 10 microns or more in diameter. The majority of aerosol particles form a thin haze in the lower level of the atmosphere (troposphere) where they tend to reduce visibility. These particles have atmospheric residence times of order a week before they are washed out of the atmosphere by rain. Depending on their composition, these aerosols can chemically react with the rain lowering its pH and producing acid rain. Aerosols are also found in the stratosphere (layer above the troposphere).

Volcanic eruptions, such as Mount Pinatubo in the Philippines in 1991 can put large amount of ash and sulfur dioxide into the stratosphere. The sulfur dioxide chemically reacts to produce aerosols. Since it does not rain in the stratosphere, these aerosols have residence times of order months. Their visual effect is to produce colorful sunrises and sunsets. Their climatological effect is to cool the surface tempertaure of the Earth. The aerosols produced by the eruption of Mount Pinatubo cooled average global temperatures over the following year by about 1°F.

Some aerosols are produced naturally, originating from volcanoes, dust storms, forest and grassland fires, living vegetation (pollen, spores, organic molecules), and sea spray. Human activities, such as the burning of fossil fuels and agriculture (over-cultivation and soil erosion), also generate aerosols. Since large particles are rapidly removed from the atmosphere by gravitational settling, they do not travel far from their sources. On the other hand, small particles, aerosols with sizes less than one micron, can travel far from their sources. For example, fine dust particles from the Sahara/Sahel region have been measured in Bermuda.

Aerosols exert their effect on climate by radiative forcing. A radiative forcing is basically a change that is imposed on the planetary energy balance that can alter the global temperature. Greenhouse gases, for example, intercept some of the outgoing radiation and thereby act to force the earth's surface temperature to come to a higher equilibrium temperature, whereas aerosols normally reflect incoming solar radiation and therefore tend to force the climate to a colder equilibrium temperature.

However, in contrast to greenhouse gases, which act primarily on the outgoing infrared radiation, aerosols can influence both sides of the energy balance. Sub-micron aerosol particles are highly effective at scattering solar radiation, sending a substantial portion back to space (reducing the amount of sunlight reaching the surface), and consequently cooling the earth. On the other hand, organic aerosols and soot, absorb radiation, and thus tend to warm the planet. In addition to this direct (scattering-absorption) effect there is also an indirect aerosol effect. The indirect effect deals with the fact that tropospheric aerosols have a substantial impact on the size distribution of cloud droplets, thus altering the radiative properties of clouds (increasing their reflectivities), and may also inhibit rainfall by potentially altering the lifetimes of clouds. This indirect appears to have a greater impact on global climate.

In addition to their effect on visibility and climate and the role they play in the production of acid rain, aerosols can also impact human health. A positive, significant relationship has been found between particulate pollution and nontraumatic deaths (as well as deaths from respiratory and cardiovascular problems). On average, a 100 microgram increase in total suspended particulates leads to a 6% increase in mortality.

Research Objectives

While aerosols are a global problem, we will begin with a comprehensive investigation of aerosols, and some of their effects, in NYC. The project addresses the following questions:

• What are aerosol optical depths in NYC and how spatially variable are they?
• Is there a relationship between aerosol and asthma?
• What are the trends in aerosols and aerosol precursor species?
• What are the trends in acid rain and how are they related to the trends in aerosol?
• What is the relationship between EPA-sampled particulates and aerosol optical depths?
• What are the implications for the future?

Research Tasks

• Build a hand-held sunphotometer and make measurements throughout the day. The data will be recorded onto a web-page and placed in the archive.
• Build an additional hand-held sunphotometer that will be distributed to other students who will make measurements in coordination with the team.
• Characterize the spectral response of the LED used in the hand-held sunphotometer. Product: spectral response curve for LED.
• Analyze the sunphotometer data using the Langley Method and Excel. Product: Time series of aerosol optical depths and Io.
• Perform a correlative analysis of EPA acid rain and aerosol data (particulates and precursor species). This study may be supplemented with the analysis of long term data obtained at Black Rock and/or Mohonk Lake. Products: pH trends, aerosol trends and correlations between them.
• Obtain data on asthma incidence and examining the relationship between asthma and aerosol through a statistical analysis of the asthma and aerosol data. Products: Asthma statistics, correlation between asthma and aerosol.
• Examine the relationship between pollen counts and aerosol load (either optical depth or particle counts). Products: Times series of pollen counts, time series of aerosol optical depth or counts and relationship between them.
• Analyze EPA particulate and precursor gas data for trends. Products: Graphs of trends.
• Examine the relationship (looking for correlations) between EPA data and aerosol optical depth measurements. Product: Correlation between particulates and aerosol optical depth. Issue: Sampling.
• Analyze the sunphotometer data looking for spatial variations within NYC and relationships to meteorological variables. Products: Time series of aerosol optical depth and meteorological variables and their correlations.

Campus-based Research Team Tasks

LaGuardia Community College:

1. make side-by-side polarimeter measurements to examine the reproducibility of the data
2. analyze exiting polarimeter data as well as the data obtained over the summer to retrieve aerosol optical thickness, particle size and refractive index.
3. set-up the LaGuardia Community College weather station
4. examine the effect of modifications to the polarimeter design (e.g., construct a polarimeter with a telescope in the front end to see if this increases the signal)
5. characterize the spectral response of the polarimeter filters
6. examine the relationship between the polarimeter and sunphotometer retrieved aerosol optical depths.

Medgar Evers College:

1. Ensure that the MFRSR at Medgar Evers is properly aligned and collecting data.
2. Arrange for the MFRSR data to be archived in the SIRN archive.
3. Learn how to analyze the MFRSR data using GISS-developed analysis tools.
4. Quantitatively compare the hand-held and MFRSR results.